1. Field of the Invention
The present invention relates to spring scales and more particularly to a mass producible scale having an optical detector.
2. Brief Description of the Prior Art
Optical detection and magnification of deflection has been employed for the purpose of precise measurement in conjunction with spring weighing scales to provide accurate and simplified readings. Typical with prior optical detectors used in conjunction with such scales has been the use of Rhonchi rulings for generating optical interference patterns. In general, such detectors comprised a pair of optical gratings, one movable with the scale load and support and the other fixed to the scale frame. Movement of the scale load support resulting from the placement of a load on a scale pan provided modulation of a light beam passing through the grating sets. With one grating set having its gratings skewed to the other, a moire interference pattern was generated. With both gratings parallel, the movable grating set acted as a shutter to modulate the light beam at a detectable rate with each light to dark transition indicating a displacement equal to a grating line spacing.
With either approach many problems were inherent with the use of optical gratings for detection purposes. For example, alignment of such optical systems was difficult and costly because clearance between the movable and fixed gratings had to be precisely controlled.
In U.S. Pat. No. 3,923,110 a spring scale having an optical detector is disclosed. The detector included a somewhat complex guiding mechanism for the purpose of accurately controlling the movement of a movable grating in a plane spaced from a fixed grating. Unfortunately, the guiding structure introduced further inaccuracies into the system by providing frictional losses between grating guides and the movable grating. Further, a flexible link interconnected the movable grating with the scale tare, and the employment of such link introduced additional error to the system.
It may be appreciated that the difficulty in achieving proper spacing between the grating sets and further optical grating alignments presented a significant drawback to production output of optical detection scales, because significant skilled labor was necessary. In production, a technician would be required to effect optical alignment. When gratings were separated by too great a distance, the resolution of the detector was sacrificed, while placement of the gratings too close to one another resulted in interference during load support structure movement. Further, it was necessary to assure that the grating sets lay within parallel planes to maintain integrity of resolution over the load support travel.
A scale having an optical detector which utilized skew gratings to provide a moire interference pattern was disclosed in a copending application: weighing with Moire Optoelectronic Transducer, Ser. No. 653,850, filed Jan. 30th, 1976, and assigned to the assignee of the present invention. In order to assure proper grating registration and control the travel path of a movable grating a plurality of guide rails were provided. Such guiding structure was not well suited to mass fabrication and increased scale costs, yet still required significant skilled labor to effect grating alignment.